1. Field of the Invention
The invention relates generally to the configurations and methods of manufacturing the semiconductor devices. More particularly, this invention relates to a gallium nitride (GaN)-based field effect transistor implemented with new device configurations and manufacturing methods for a normally-off GaN-based field effect transistor that has an extremely small ON resistance in conducting a large amount of electric current.
2. Description of the Prior Art
Conventional methods of configuring and manufacturing a gallium nitride (GaN) based field effect transistors (FETs) are still continuously challenged with a technical issue for providing a normally-off FET transistor that has simple and convenient manufacturing and operating configurations. Specifically, gallium nitride (GaN) based FETs have been implemented to make high electron mobility transistors (HEMTs). For applications of the power transistors, this type of transistor may replace some power devices implemented with power metal-oxide-semiconductor (MOS) field-effect transistor using a silicon-based semiconductor that are commonly and widely used now. Compared to the silicon-based MOS-FET (or MOSFET) semiconductor power devices, the GaN-based transistors can further reduce the on-resistance and realize higher breakdown voltage by taking advantage of the semiconductor material characteristic of a wide band-gap. Furthermore, this high electron mobility transistor can also provide high speed switching and high sensitivity operations relative to the performance of the silicon-based MOSFET devices.
The basic principle of achieving the high electron mobility is achieved by bonding two different kinds of semiconductor materials with different band gaps. A two dimensional electron gas (2DEG) layer is generated at the interface thus serving as a current path comprising a flow of electrons in this electron gas layer. A specific example is illustrated in FIG. 1 with an aluminum gallium nitride (AlGaN) epitaxial grown on top of gallium nitride (GaN) layer. With different band gaps of these two materials, a two dimensional electron gas layer (2DEG) is generated in the boundary, referred to as an AlGaN/GaN hetero-junction, between these two semiconductor materials. Typically, the AlGaN/GaN hetero junction structure is supported on insulating substrate, such as a sapphire substrate. The transistor further includes a source electrode S and a drain electrode D which are arranged on two opposite sides of a gate electrode G formed onto the AlGaN layer that spreads between the source electrode S and the drain electrode D.
With the AlGaN layer functioning as an electron supply layer and supplies electrons to the 2DEG in the undoped GaN layer, the electrons in the electron gas layer transmits between the source electrode and the drain electrode even when there is no control voltage applied to the gate. The high electron mobility transistor (HEMT) configuration as that shown in FIG. 1 thus operates in a normally-on mode unless a voltage is applied to the gate to pinch off the current flow between the source and the drain electrode. A requirement to continuous apply a pinch off voltage to the gate in order to turn off the transistor thus leads to additional power consumptions and may often cause a more complicated device control process for implementing such transistor in an electronic device. In addition most applications are designed for normally-off transistors and so this device would not be suitable to those applications. For these reasons, it is desirable to provide new and improved configurations for manufacturing GaN-based transistors such that the device is normally-off, without requiring application of a pinch-off voltage to the gate.
Most of the AlGaN/GaN hetero structure field effect transistors (HFETs) are provided as depletion mode metal-semiconductor FET (MESFET) in order to achieve a low on resistance RdsA (drain-to-source resistance*area). Enhancement mode MESFET devices with threshold voltage Vth between 0.3 to 0.7 volts have been disclosed. But these types of transistors cannot be driven by a gate voltage between the traditional gate voltage of ten to fifteen volts. Also, various efforts have been attempted to build enhancement mode metal insulator semiconductor FET (MISFET) on a p-GaN layer using different gate dielectrics including silicon nitride (Si3N4), silicon oxide (SiO2) and gadolinium oxide (Gd2O3). However, such devices suffer the disadvantages of low inversion mobility and a very high electric field in the oxide when the device is biased into the breakdown thus causing device reliability concerns. In order to address this issue, an oxide layer with increased thickness had been implemented, but that degraded the transconductance and lead to an undesirable higher RdsA.
For all these reasons, there are great and urgent demands to improve the device structure with low RdsA while not disturbing the conductivity of the two-dimensional electron gas (2DEG) layer. In the meanwhile, it is desirable that the device may be operated as a normally off device without applying a voltage to the gate such that the above-discussed difficulties and limitations may be resolved.